Chromatography is the primary technique for separating complicated mixtures of chemical compounds. This is achieved by passing a sample in a mobile phase through a column containing a stationary phase. Chromatography is distinguished into two branches—gas chromatography (“GC”) and liquid chromatography (“LC”), depending on the physical state of the mobile phase. Two types of columns may be used in both gas and liquid chromatography—packed and capillary. Packed columns consist of a tube filled with packing material. The stationary phase is applied to the surface of the packing material. In capillary columns, the stationary phase is applied directly onto the inner wall of the capillary. Capillary columns offer substantial advantages over packed columns.
In gas chromatography, the vast majority of analyses are conducted using capillary columns. Capillary columns for gas chromatography are made from single wide capillaries (about 0.5 mm inner diameter) compatible with standard chromatographic equipment. The columns are further differentiated based on the number of capillaries contained therein. Monocapillary columns consist of a single tube, while multicapillary columns comprise many tubes, allowing for a high sample capacity.
The diffusion of molecules in gases is very fast, due to the low viscosity. Compared to gases, liquids are much more viscous. The diffusion of molecules in liquids is 10,000–100,000 times slower than in gases. To provide rapid mass-transfer, the diameter of capillary columns used for liquid chromatography must be very small, usually less than 20 μm. Due to their minuscule sizes and very low sample capacity, capillary columns for liquid chromatography made from single capillaries are not compatible with standard equipment and cannot be used for routine liquid chromatography analyses.
Methods for the preparation of insoluble stationary phases for liquid chromatography are known. Most of these methods are based on chemical bonding of the stationary phases to the surface of chromatographic supports. However, the use of multicapillary columns in liquid chromatographic applications has been constrained because of technical problems. The primary constraint is that the linear flow rate of the mobile phase in wider capillaries is higher than the flow rate in narrower capillaries. If the phase ratio, i.e. the ratio of the radius of the capillary to the stationary phase film thickness, is the same for all capillaries, the sample will move faster in the wider capillaries. As a result, the efficiency of multicapillary columns is always inferior to that of single capillary columns.
Accordingly, there is a need for an efficient multicapillary column having a high sample capacity, for use in liquid chromatography.